A reaction cell for automatic biochemical analyzer in which weld generation in beam transmission parts is prevented to reduce scattering of transmitted beam, thereby having a stable transmissivity to achieve high analytical efficiency is provided. It is a reaction cell which is bottomed and has an opening formed on one end, the reaction cell comprising a tube wall including one pair of walls facing to each other and two side walls each connecting to each of the one pair of walls via a corner portion, wherein the one pair of walls each have a thickness larger than thicknesses of the corner portions, and have a uniform thickness over the entire wall, or when each wall has a maximum value in thickness in a part of the wall, the thickness monotonically decreases from the part having the maximum value to the corner portion.
